Borehole seismic data can be utilized to refine surface seismic data prior to drilling production wellbores. Borehole seismic data can further be gathered on a continuing or recurrent basis to monitor subsurface formations and reservoirs during production of the well. The gathering of seismic data on a continuing basis facilitates extraction of gas or oil deposits.
Borehole seismic surveys are usually conducted by placing seismic receivers in a borehole and firing a seismic source at the surface to generate an acoustic wave. The receivers are often placed in a shuttle and deployed downhole for the duration of the survey. The receivers are generally retrieved following the survey. The amount of information that can be obtained in borehole seismic surveys is sometimes limited by the logistics of deploying the shuttles downhole.
As borehole surveys have advanced, the number of receivers and the distance between receivers and sources has generally increased to improve the ability to detect formation characteristics in the undisturbed formation at increasing distances from the borehole. Increasing the distance between the source and the receivers results in deeper seismic wave penetration. Therefore the receivers detect signals that are representative of conditions at greater distances from the borehole. Nevertheless, increasing the distance between sources and receivers also requires an increased tool length, and longer tools can cause difficulties with deployment. Increased distances between sources and receivers also result in longer logging periods, more down-time for the drilling rigs, and higher costs.
A downhole seismic logging tool for generating a seismic survey may include a proximal logging head, telemetry electronics, data acquisition electronics, and an array of shuttles interconnected by flexible cables. The logging head, and the sections for telemetry electronics and acquisition electronics are generally quite heavy and connected in series. The shuttles in the array, on the other hand, are usually small and relatively light for better acoustic performance. The entire logging tool is usually deployed by an armored logging cable connected by the logging head. The tension on the logging cable is typically monitored by a surface cable winch.
There are substantial risks involved when advancing and retracting the array of shuttles into and out of a wellbore. For example, if a shuttle is obstructed when a logging tool is advanced downhole, the interconnect cable tends to collect on the obstructed shuttle. If an operator notices an obstructed shuttle (by monitoring tension at the uphole winch), he can stop the winch and attempt to avoid cable tangling, but many times an obstructed shuttle is generally difficult to notice until the cable becomes hopelessly entangled, especially when the number of shuttles is large, and the weight of a shuttle is small compared to the entire weight of the array, small change in conveyance tension is difficult to detect.
To avoid shuttle encumbrances, shuttles are sometimes locked radially inward by mechanical arms, springs, or magnetic clamps. However, shuttles are often released and stationed at a single location for a period of time while measurements are taken. When the shuttles are stationary, the cables can become trapped, by, for example, a cable being pulled into the mud cake as a result of differential pressures between the borehole and an adjacent formation. The differential sticking can occur for hundreds of meters of cable such that the logging tool below the stuck cable can not be retrieved.
As mentioned above, cable tension is often monitored at surface as a tool is deployed downhole. However, a surface tension measurement is far from fail-safe. Logging tool cables are generally long and heavy, and therefore the surface tension is ever-increasing. Small changes in tension can easily go undetected until well after excess cable accumulates over an obstructed tool. After a deployment of any significant distance, the cable is much heavier than a downhole tool, and the surface tension changes are dominated by the weight of the lengthening cable. Only a very small fraction of the surface tension measured will be a result of an obstructed tool. Accordingly, a stuck tool may result in a tangled cable, which makes tool recovery difficult.
If a downhole logging tool gets stuck, one way to recover the tool is by conventional “fishing.” However, it is important to know what section of the tool is stuck in order to plan a recovery operation. Generally, there are two ways to fish out a downhole tool. One way is to put the cable in tension and increase the tension above the capacity of a pre-designated weak point at the logging head known as an overpull operation. The weak point breaks above a certain tension, and once broken the cable can generally be removed from the wellbore. Following removal of the cable, an “overshot” may be send downhole. An “overshot” may be a series of connected drill-pipes with a distal grabber. When the overshot reaches to the logging head, it grabs the tool. Drill-pipe is generally much stronger than cable, and the drill-pipe is retracted, usually bringing the tool with it. In the case of a tool having an array of sensor shuttles, connected by flexible cable, below the logging head and telemetry section, if the head and telemetry sections are not anchored, the overpull operation causes the head and telemetry sections to drop onto the sensor array such that a fishing operation is not possible over the tangled cable.
Another way to begin a downhole tool recovery operation is to cut the logging cable at the surface. Generally the cable is clamped at the wellhead prior to cutting. A drillstring is then run over the cable, and the drillstring eventually grabs and retrieves the stuck tool. This process is often referred to as “cut and thread,” and is very time consuming.